FIG. 1 shows a conventional combined cycle plant with a gas turbine cycle 1 and a water/steam cycle 2. In the gas cycle 1, air comes into the air compressor 10 and is mixed with fuel F in the combustion chamber 11 and burned. The combustion products are then fed into the turbine section 13 causing the turbine shaft 14 to rotate the generator 15 which generates electricity.
Exhaust E from the gas turbine 13 enters the transition duct 19 that leads to the HRSG 20 and is cooled as it passes over the water/steam circuit, heating and boiling the water to steam. After it has given up the practical amount of energy (cooled) to the water/steam cycle, it is exhausted out the HRSG stack 21. It is here, in the exhaust stack, where emissions are measured for reporting to the Environmental Protection Agency (EPA) and determining emissions compliance.
The water/steam cycle 2 consists of the HRSG 20, the Steam Turbine (ST) 70, a generator 75, and a condenser 76. The ST 70 consists of a high-pressure (“HP”) section 71, an intermediate-pressure (“IP”) section 72, and a low-pressure (“LP”) section 73. Illustrated is a three pressure Reheat system, also allocable are three Pressure Non-Reheat, two Pressure and one Pressure HRSGs and Boilers. The IP section is sometimes referred to as the reheat turbine. The three ST sections 70, and the generator 75, are all on a common shaft 74. When the steam flows through the ST 70, it turns the shaft 74 and electricity is generated by the generator 75. This steam exits the ST 70 and flows to the condenser 76.
As also shown in FIG. 1, a conventional combined cycle plant typical includes additional conventional elements, for example, low pressure condensate 22, low pressure (LP) economizer 23, LP steam drum 31, LP downcomer 32, LP evaporator 33, LP superheater 34, pipe to LP section (73) of ST 35, LP bypass valve & de-superheater 37, IP economizer 40, IP steam drum 41, IP downcomer 42, IP evaporator 43, IP superheater 44, pipe to cold reheat pipe (61) 45, HP economizer 50, HP economizer 51, P downcomer 53, pipe to HP section (71) of ST 56, HP bypass Valve & de-superheater 58, cold reheat pipe 61, hot reheat pipe to IP section (72) of ST 63, RH bypass valve & de-superheater 65 and boiler feed pump 81, the operations of which are known and therefore not discussed in further detail.
During normal operation between full load and some minimum load, all the steam produced in the HRSG goes to the ST without pressure control (sliding pressure) and exits the ST LP section 73 into the condenser 76 where it is condensed into water to cycle back through the water/steam cycle 2 starting at pump 80.
The steam exiting the HRSG flows through valves [HP: 57, RH: 64 and LP: 36] going to the ST (ST inlet valves) to generate power as stated above. These valves can be used to control the pressure in water/steam circuit in their respective pressure levels if necessary under certain operating conditions.
The traditional configuration is to place all of this emissions control equipment downstream of the HP boiler drum in the exhaust stream. Shown in FIG. 1 is a medium temperature CO catalyst 130, then an ammonia injection grid 120, and finally a SCR Catalyst 110 for NOx control.
However, typical gas turbine power plants (GTPP), for example, the conventional combined cycle plant shown in FIG. 1, are typically designed to operate between 50% to 100% load to generate power while maintaining permit emissions compliance levels. Sometimes the plants include Duct Firing to gain capacity above the normal 100% load range. These levels are controlled by utilizing industry standard emissions control equipment. Below approximately 50% load, the gas turbine (GT) emissions can increase dramatically, over a factor of 300. See Table 1A to illustrate the local minimum. Note that there is a local minimum of emissions at 17% GT load in this example.
TABLE 1AGT Exhaust Emissions vs. GT LoadGT Load (%)NOxCOVOCNO/NOx (%)1001510279501510272402785010052035300012005173012002805540280015005Note:Values in ppmvd @ 15% O2 unless noted
This translates into traditional stack emissions as indicated in Table 1B for NOx, CO, and VOC. The upper load range from 100% load to approximately 50% load is where emissions compliance is achievable. The 50% load is where the emission from the GT are at a low enough level that the traditional post combustion clean up equipment can destroy enough emissions to have compliant levels at the stack. This 50% load point is often referred to as Minimum Emissions Compliant Load (MECL) for a gas turbine. The unit is out of emissions compliance at GT load ranges<approximately 50% load. This is a result of the engine characteristics at lower GT loads: a large increase in emissions and a large increase in the NO2 portion of NOx.
TABLE 1BStack Exhaust Emissions vs. GT Load, Existing TechnologyGT Load (%)NOxCOVOC10022250222401345602016150650171360160518150830Note:Values in ppmvd @ 15% O2 unless noted
NOx emissions are comprised of NO and NO2. As GT load decreases below 50%, the constituents of the NOx shift from NO to NO2 which is much more difficult to destroy. Selective Catalytic Reduction (SCR) De-NOx reactions, in which NOx is reduced into nitrogen by NH3, generally progress according to reaction (I) below. In cases where NO2 coexists with NO, reactions (2) and (3) occur. If the percentage of NO2 in NOx is less than 50% (GT loads≧50%), NO2 and NO are removed by reaction (2), and the NO that remains is removed by reaction (1). If the percentage of NO2 in NOx is higher than 50% (GT loads<approximately 50%), NO2 in the remaining NOx component becomes rich as reaction (2) progresses. Under this circumstance, the De-NOx reaction drops significantly because reaction (3) is slow.4NO+4NH3+O2=>4N2+6H2O  (1)NO+NO2+2NH3=>2N2+3H2O  (2)6NO2+8NH3=>7N2+12H2O  (3)
Traditional GTPP's may have an oxidation (CO) catalyst 130, an ammonia injection grid 120 and a SCR (NOx) catalyst 110 for controlling the emissions in the GT exhaust path, typically where the temperature is in the range of 500° F. to 700° F. (although somewhat higher temperatures are typical for gas turbine simple cycle plants). This equipment is typically located downstream of the HP evaporator tubes in combined cycle applications, where the temperature of the exhaust is appropriate for these chemical reactions to occur effectively. The traditional SCR catalyst 110 is designed to destroy relatively high concentrations of NO, and relatively low quantities of NO2.